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The ferns are an ancient lineage of vascular
plants, dating back to at least the
Devonian. They include three living
groups: Marattiales, Ophioglossales, and leptosporangiate ferns.
There are also two early groups now extinct: Stauropteridales (known
from the Upper Devonian and Carboniferous) and Zygopteridales (the
oldest group of fossil ferns). An additional group, the
Psilotales,
is now tentatively included in the ferns. Though the group is so vastly
different in appearance from living ferns, genetic and developmental evidence
is accumulating in favor of its classification with the ferns.

Major groups within the ferns are classified based on the structure and
location of their sporangia. Most modern ferns produce their sporangia
on the underside of their leaves, but most early ferns (and some living groups)
produced them along their stems or on specialized stalks that do not look much
like leaves at all.

The ferns are now considered close relatives of the horsetails, a small and bizarre-looking group of plants.
Both are believed to have evolved from a group of Devonian plants that
includes the Cladoxylopsida.